Amorphous PEI polymers are broadly used as super engineering plastics, as film materials, or as injection-molding materials in various fields, such as electrical and electronic component fields, and automobile part fields, because they are excellent in physical property, fire retardancy, heat-resisting property, mechanical property, insulation, and melt processability.
For example, Patent Document 1 discloses a PEI film obtained by stretching PEI at a sufficiently lower temperature than the glass transition temperature of the PEI, and describes that the obtained PEI film is excellent in initial modulus and breaking strength.
In general, it is difficult to form fibers from amorphous PEI polymers. Amorphous molecules randomly existing in the amorphous PEI polymer make it difficult to form an oriented structure generally required for fibers. Therefore, even if an amorphous PEI polymer is subjected to form fibers therefrom, such obtained fibers generally hardly satisfy the quality for practical use. In fact, although Patent Document 1 exemplifies a yarn as an embodiment of molded article, Patent Document 1 does not actually produce yarn in any of the Examples.
Accordingly, Patent Document 2 proposes a method for producing a PEI fiber by drawing an as-spun PEI yarn without using oil solution, the as-spun PEI yarn being obtained by melt spinning method. Patent Document 2 describes that the tenacity of thus obtained PEI fiber can be improved by the above-mentioned drawing method.
Moreover, when melt spinning of amorphous PEI polymer, the temperature required for the melt spinning method is almost 400° C., which is close to the decomposition temperature of the polymer. Therefore, the method has the problem that volatile component is easy to generate from the polymer in the melt spinning process. In view of this, a method for producing of an amorphous PEI fiber comprising melt spinning of an amorphous PEI polymer is also proposed. In the method the water content of the polymer is controlled in the extruder or a volatile component is deaerated from the extruder in order to accomplish PEI fiber formation by using melt spinning method (see, for example, Patent Document 3).